1. Field of the Invention
The present invention relates to an optical information recording and reproducing device such as an optical pickup suitably used for a compact disk reproducing unit and the like. More particularly, the present invention relates to an optical information recording and reproducing device having a detecting system for generating servo signals.
2. Description of the Related Art
An optical disk can store a huge amount of information with high density. In recent years, the applications for such an optical disk have been developed in various fields. The optical disk can be classified into a rewritable type, a write once type, and a reproduction only type depending on the erasability of recording pits formed thereon. An optical information recording and reproducing device is used to record information on such an optical disk and to reproduce information recorded thereon. Such a device can be classified into a phase change type, a reflectance change type, etc. depending on the method of forming recording pits on the optical disk. Generally, in the optical information recording and reproducing device, a recording medium such as the optical disk has a guide groove corresponding to a recording track previously formed thereon so as to allow light beams to trace the guide groove under tracking control when information is recorded, reproduced, or erased on or from the recording medium.
In the tracking control, a tracking error is conventionally detected mainly by two methods: a one-beam "push-pull method" and a "three-beam method".
First, referring to FIGS. 7 to 10, the one-beam push-pull method will be described. In this method, a light beam used for both recording and reproduction is radiated onto a track on an optical disk as the recording medium. The light beam reflected from or passing through the optical disk is used as detecting light. The detecting light is introduced to a diffraction element having two areas divided by a division line running parallel to the direction of the track on the optical disk (hereinafter referred to as the "track direction"). The light beams diffracted from the two areas are separately inspected by an optical detector, where the difference of light amounts is detected as a tracking error.
FIG. 7 shows a conventional optical pickup using the one-beam push-pull method for detecting a tracking error. Referring to FIG. 7, a light beam (divergent light) emitted from a semiconductor laser 101 passes through a diffraction element 102 and is converged on an optical disk 105 through a collimator lens 103 and an objective lens 104. The light beam reflected from the optical disk 105 returns through the objective lens 104 and the collimator lens 103 and is diffracted with the diffraction element 102 so as to be converged on an optical detector 106.
Then, a focusing error detection mechanism and a tracking error detection mechanism of this optical pickup using the one-beam pull-push method will be described.
FIGS. 8A and 8B show the details of the diffraction element 102 and the optical detector 106, respectively. These figures also show the relative positions to each other of the diffraction element 102 and the optical detector 106 as seen from top.
First, the focusing error detection mechanism will be described. As is shown in FIG. 8A, the diffraction element 102 is substantially circular and has two semicircular regions 102a and 102b divided by a division line PL. As is shown in FIG. 8B, the optical detector 106 has four detecting portions 106a, 106b, 106c, and 106d divided by division lines A', B', and C'. One part of the returned light beam incident to and diffracted from the region 102a of the diffraction element 102 is converged on the division line A' as a converged area 108a, and the other part incident to and diffracted from the region 102b is converged on the division line C' as a converged area 108b.
In the above-described configuration, when the light beam emitted from the semiconductor laser 101 is accurately converged on a spot on the optical disk 105 through the objective lens 104, the converged areas 108a and 108b are formed as spots right on the division lines A' and C', respectively, as is shown in FIG. 9B. As a result, the light amounts on the detecting portions 106a and 106b and the light amounts on the detecting portions 106c and 106d are equal to each other, respectively.
On the other hand, in cases where the optical disk 105 is moved closer to the objective lens 104 due to some cause, the focal points of the diffracted beams are formed behind the optical detector 106. As a result, as is shown in FIG. 9C, the converged areas 108a and 108b are formed not on the division lines A' and C' as a focal point, but on the detecting portions 106a and 106b in a semicircular shape, respectively.
In cases where the optical disk 105 is moved farther from the objective lens 104 due to some cause, the focal points of the diffracted beams are formed in front of the optical detector 106. As a result, as is shown in FIG. 9A, the converged areas 108a and 108b are formed not on the division lines A' and C' as a focal point, but on the detecting portions 106a and 106b in a semicircular shape, respectively.
Thus, a focusing error signal FES output from the optical detector 106 is obtained by calculating the following equation: EQU FES=(S1+S4)-(S2+S3) (1)
wherein S1, S2, S3, and S4 are output signals from the detecting portions 106a, 106b, 106c, and 106d of the optical detector 106, respectively. The calculation is performed with adders 110a and 110b and a subtractor 111 as is shown in FIG. 8B.
Next, the tracking error detection mechanism will be described. FIGS. 10A to 10C respectively show the relative positions to each other of a converged spot 109 on the optical disk 105 and an information track (pit array) 120, together with the intensity distribution of the returned light beam. As is shown in FIG. 10B, the intensity distribution of the returned light beam is symmetrical with regard to the track direction when the converged spot 109 is right on the information track 120.
On the other hand, as is shown in FIG. 10C, in cases where the information track 120 is shifted to left with regard to the converged spot 109 due to some cause, the right portion of the returned light beam (hatched portion) is dark, while the left portion thereof is bright. Likewise, as is shown in FIG. 10A, in cases where the information track 120 is shifted to the right with regard to the converged spot 109, the left portion of the returned light beam (hatched portion) is dark, while the right portion thereof is bright.
As is shown in FIG. 8A, the returned light beam is split into two beams in accordance with the regions 102a and 102b of the diffraction element 102, and the division line PL thereof is parallel to the track direction. Thus, a tracking error signal TES output from the optical detector 106 is obtained as the difference of the light amounts of the converged areas 108a and 108b, which is obtained by calculating the following equation: EQU TES=(S1+S2)-(S3+S4) (2)
wherein S1, S2, S3, and S4 are output signals as defined earlier. The calculation is performed with adders 112a and 112b and a subtracter 113 as is shown in FIG. 8B.
Thus, based on the focusing error signal FES and the tracing error signal TES obtained as described above as servo signals, the objective lens 104 is properly driven with an actuator (not shown) so that the converged spot 109 can be placed right on the information track 120.
Next, referring to FIGS. 11 to 14, the three-beam method will be described. In this method, a light beam is split into a main beam and two sub-beams by means of a diffraction element, and any tracking error is detected using the sub-beams.
FIG. 11 shows a conventional optical pickup using the three-beam method. Referring to FIG. 11, a light beam emitted from a semiconductor laser 201 is introduced into a first diffraction element 207 where the incident light beam is split into a zero-order diffracted beam (main beam) and first-order diffracted beams (sub-beams) for detecting any tracking error. The three diffracted beams pass through a second diffraction element 202, and are converged on an optical disk 205 through a collimator lens 203 and an objective lens 204. The light beams reflected from the optical disk 205 return through the objective lens 204 and the collimator lens 203 and are diffracted with the second diffraction element 202 so as to be converged on an optical detector 206.
Then, the focusing error detection mechanism of the optical pickup using the three-beam method will be described.
FIGS. 12A and 12B show the details of the second diffraction element 202 and the optical detector 206, respectively. These figures also show the relative positions to each other of the second diffraction element 202 and the optical detector 206 as seen from top.
As is shown in FIG. 12A, the second diffraction element 202 is substantially circular and has two semicircular regions 202a and 202b divided by a division line DL. As is shown in FIG. 12B, the optical detector 206 has five detecting portions 206a, 206b, 206c, 206d, and 206e divided by division lines A", B", C", and D".
One part of the returned main beam incident to and diffracted from the region 202a of the second diffraction element 202 is converged on the division line A" as a converged area 208a, and the other part incident to and diffracted from the region 202b is converged on the detecting portion 206d as a converged area 208b, as is shown in FIG. 12B. On the other hand, one of the returned sub-beams forms converged areas 208a' and 208b' on the detecting portion 206a. Likewise, the other returned sub-beam forms converged areas 208a" and 208b" on the detecting portion 206e.
In the above-described configuration, when the light beam emitted from the semiconductor laser 201 is accurately converged on a spot on the optical disk 205 through the objective lens 204, the converged area 208a is formed as a spot right on the division line A", as is shown in FIG. 13B. As a result, the light amounts on the detecting portions 206b and 206c are equal to each other.
On the other hand, in cases where the optical disk 205 is moved closer to the objective lens 204 due to some cause, the focal points of the diffracted beams are formed behind the optical detector 206. As a result, as is shown in FIG. 13A, the converged area 208a is formed not on the division line A" as a focal point, but on the detecting portion 206b in a semicircular shape.
In cases where the optical disk 205 is moved farther from the objective lens 204 due to some cause, the focal points of the diffracted beams are formed in front of the optical detector 206. As a result, as is shown in FIG. 13C, the converged area 208a is not formed on the division line A" as a focal point, but on the detecting portion 206c in a semicircular shape.
Thus, the focusing error signal FES output from the optical detector 206 is obtained by calculating the following equation: EQU FES=S2-S3 (3)
wherein S2 and S3 are output signals from the detecting portions 206b and 206c of the optical detector 206, respectively. The calculation is performed with an adder 210 as is shown in FIG. 12B.
Next, the tracking error detection mechanism will be described. FIGS. 14A to 14C respectively show the relative positions of converged spots 209, 209', and 209" on the optical disk 205 and an information track 220. As is shown in FIG. 14B, the converged spots 209' and 209" formed by the sub-beams are located apart the same distance away from the converged spot 209 formed by the main beam in the opposite directions to each other along the information track 220. Further, the converged spots 209' and 209" are slightly shifted with regard to the information track 220 in the opposite directions to each other.
In cases where the information track 220 is shifted to left with regard to the converged spot 209 due to some cause, as is shown in FIG. 14A, the converged spot 209' is located substantially right on the information track 220. This results in that the intensity of the reflected light beam from the converged spot 209' decreases. At this time, the converged spot 209" is further shifted from the information track 220, so that the intensity of the reflected light from the converged spot 209" increases. Likewise, in cases where the information track 220 is shifted to right with regard to the converged spot 209, the converged spots 209' and 209" are shifted in reverse as is shown in FIG. 14C. This results in that the intensity of the reflected light from the converged spot 209' increases and that of the reflected light from the converged spot 209" decreases.
As described earlier, the sub-beams reflected from the converged spots 209' and 209" are converged on the detecting portions 206a and 206e of the optical detector 206. Accordingly, the tracking error signal TES is obtained by calculating the following equation: EQU TES=S1-S5 (4)
wherein S1 and S5 are output signals from the detecting portions 206a and 206e. The calculation is performed with a subtracter 211 as is shown in FIG. 12B.
Thus, based on the focusing error signal FES and the tracking error signal TES obtained as described above as servo signals, the objective lens 204 is properly driven with an actuator (not shown) so that the converged spot 209 be placed right on the information track 220.
The above-described three-beam method can provide stable focusing and tracking detection without being affected by the inclination of the optical disk 205 nor the depths of pits and a guide groove thereof. Accordingly, this method is mainly employed for an optical pickup used for an optical disk of the reproduction only type.
However, both of the above methods have a problem when applied to an optical pickup capable of recording on and/or reproducing from three types of compact disks (CDs), i.e., the rewritable type, the write once type, and the reproduction only type.
The problem arises because an optical disk has a constant linear velocity. In the reproduction only type optical disk, the recording information thereof includes information for velocity control, so that the rotational velocity of the optical disk can be controlled upon the start of the reproduction. However, in the rewritable type or write only type optical disk, no information is recorded thereon at an initial recording, so that velocity control as above is not possible.
To overcome the above problem, a guide groove is provided for the rewritable type and write once type optical disks, as is shown in FIG. 15, and such a guide groove 230 is wobbled at a predetermined period. Thus, the linear velocity of the optical disk is kept constant by detecting the periodic wobbling. Based on the standard for the CDs in which the linear velocity is 1.2-1.4 m/sec. and the frequency of the wobbling is 22.1 kHz, period L of the wobbling of the guide groove 230 is 54-63 .mu.m.
In order to detect the wobbling with high sensitivity by an optical pickup using the three-beam method, the distance between the two converged spots of the sub-beams should be L.times.N (N is an integer). When the above value for L is used, the distance is approximately 60 .mu.m, 120 .mu.m, . . . . Because of the restriction in the optical design, the distance is limited to 60 .mu.m, thus preventing the optical pickup from being designed freely. Further, to improve the performance of the tracking, the distance should be as small as possible. At present, it is designed to be approximately 30-40 .mu.m. Moreover, because of the restriction in the arrangement of the converged areas formed on the optical detector 206, most preferably, the division line on the second diffraction element 202 should be vertical to the track direction. This has been applied in the actual design. This restriction is inevitable for all optical pickups using a diffraction element for splitting a light beam into a main beam and sub-beams.
For the above-described reasons, the three-beam method is disadvantageous in application to the optical pickup used for the rewritable type and the write once type optical disks, thus limiting the application thereof to an optical pickup used for the reproduction only type optical disk.
On the other hand, the one-beam push-pull method is free from the restriction in the arrangement of the converged areas formed on the optical detector. Further, in general, the one-beam push-pull method can detect the wobbling with higher sensitivity than the three-beam method. However, according to the one-beam push-pull method, when the objective lens moves in the radial direction of the optical disk, the returned light beam is introduced to the diffraction element 102 with an offset 240 from the center division line as is shown in FIG. 16. Further, the one-beam push-pull method is easily affected by any inclination of the optical disk. Thus, the one-beam push-pull method is disadvantageous in the points which are advantageous for the three-beam method.
Because of the above-described reasons, there has not been realized an optical information recording and reproducing device capable of recording on and/or reproducing from the optical disks of the rewritable type, the write once type, and the reproduction only type using either of the one-beam push-pull method or the three-beam method.